Communication systems are used to transmit and deliver information to consumers in a variety of ways. These systems include satellite, cellular, and wireline networks and the information can be virtually anything, from analog to digital thereby incorporating telephony, video, data, etc. Due to signal losses inherent in the transmission of signals over great distances, amplifiers are often necessary to boost the signal level. The amplifiers ensure that signals are at the proper power level as they pass through and out of the network.
Maintaining the signals at a constant power level is important for many systems, such as video distribution systems. For cable distribution systems, if the power signal level is either too high or too low, the reception at the customer's premises will be affected. If the power signal level is too high, the signal becomes saturated and the picture is distorted. On the other hand, when the power level is too low, the signal to noise ratio drops and the picture will have poor reception. The cable distribution systems consequently need to carefully monitor the power level of the signals and make appropriate adjustments.
The performance of a communication system, such as a cable distribution network, is affected by temperature. The lines that carry the signals have losses that vary with temperature. In addition to the lines, the performance of amplifiers within a communication system varies with the temperature. A cable distribution system is especially sensitive to temperature changes since the amplifiers and the coaxial cable are at ambient temperature, which can fluctuate greatly during a day or seasonally. Some communication systems consequently have to be operable over a range of temperatures from -40.degree. C. to +60.degree. C.
An example of a communication system 100 is shown in FIG. 1. The communication system 100 includes headend equipment 105 for generating forward signals that are transmitted in the downstream direction along a communication medium, such as a fiber optic cable 110, to an optical node 115 that converts optical signals to radio frequency (RF) signals. The RF signals are further transmitted along another communication medium, such as coaxial cable 120, and are amplified, as necessary, by one or more distribution amplifiers 125 positioned along the communication medium. Taps 130 included in the cable television system 100 split off portions of the forward signals for provision to subscriber equipment 135, such as set-top terminals, computers, and televisions. In a two-way system, the subscriber equipment 135 can also generate reverse signals that are transmitted upstream, amplified by any distribution amplifiers 125, converted to optical signals, and provided to the headend equipment 105.
A simplified diagram of the amplifier 125 is shown in FIG. 2. As discussed above, the gain of the amplifier 125 is affected by temperature and the impedance of the lines, such as coaxial cable 120, also varies with temperature. The output of the amplifier 125 should remain at a constant level to provide desired signal levels to the subscriber equipment 135. In order to provide a constant output of power at the amplifier 125, the amplifier 125 performs some adjustment based on temperature.
A conventional approach for providing for thermal compensation in an amplifier 125 is shown in FIG. 2. With reference to this figure, signals from the coaxial cable 120 are first passed through a pre-amplifier 206 to boost the signal level. The amplifier 125 also includes one or more gain stages 210 for amplifying the signals to the desired power level. To adjust for temperature, the amplifier 125 includes a variable attenuator, such as a Bode circuit 208 coupled to a thermal compensation circuit 220. The Bode circuit 208 is controlled by the thermal compensation circuit 220 so that the output of the Bode circuit 208, and consequently the output of the gain stages 210, is at a desired power level.
The operation of the thermal compensation circuit 220 will be described in greater detail with reference to FIG. 3. The thermal compensation circuit 220 includes a thermistor voltage reference circuit 222, a resistor 224, and a current mirror 226. The thermistor voltage reference circuit 222 outputs a voltage that varies with the temperature. The thermistor voltage reference circuit 222 typically is comprised of a resistor ladder and a thermistor placed in the ladder. The voltage from the thermistor voltage reference circuit 222 is converted into a current by the resistor 224 and this current is input to the current mirror 226 as a control signal. As the temperature changes, the voltage output by the thermistor voltage reference circuit 222 changes as does the current supplied to the current mirror 226. The magnitude of the currents supplied to the Bode circuit 208 from the current mirror 226 therefore vary with the temperature. The Bode circuit 208 attenuates the signals received from the pre-amplifier 206 to a level determined by the currents received from the current mirror 226. Thus, changes in temperature detected by the thermistor voltage reference circuit 222 can be used to alter the amount of attenuation provided by the Bode circuit 208.
Despite the use of the Bode circuit 208 and the thermal compensation circuit 220, conventional amplifiers, such as amplifier 125, output at power levels that fluctuate with temperature. For instance, some amplifiers with thermal compensation still have fluctuations of .+-.1 dB. A need therefore exists for improved methods, systems, and circuits for providing thermal compensation in amplifiers.